This invention relates to a method and apparatus for removing contaminants from a surface. More particularly, the invention relates to the removal of contaminants from a substrate surface through the application of energy from a high-energy source while the molecular crystal structure of the surface being treated is preserved.
Surface contaminants include discrete pieces of matter that range in size from submicrons to granules visible to observation with the eye. Such contaminants may be fine dust or dirt particles or unwanted molecules comprised of elements such as carbon or oxygen. Contaminants frequently become adhered to a surface by weak covalent bonds, electrostatic forces, van der Waals forces, hydrogen bonding, coulombic forces or dipole-dipole interactions, making removal of the contaminants difficult.
In certain instances, the presence of surface contaminants renders the contaminated substrate less efficient or inoperable for the substrate's designated purpose. For example, in certain precise scientific measurement devices, accuracy is lost when optical lenses or mirrors in the devices become coated with microfine surface contaminants. Similarly in semiconductors, surface defects due to minor molecular contaminants often render semiconductor masks or chips worthless. Reducing the number of molecular surface defects in a quartz semiconductor mask by even a small amount can radically improve semiconductor chip production yields. Similarly, removing molecular surface contaminants, such as carbon or oxygen, from the surface of silicon wafers before circuit layers are deposited on the wafer or between deposition of layers significantly improves the quality of the computer chip produced.
The need for clean surfaces free of even the finest contaminants has led to the development of a variety of currently used surface cleaning methods. These known methods, however, each have their own serious drawbacks. For example, widely used chemical and mechanical cleaning techniques require the use of cleaning tools and agents that can introduce as many new contaminants to a treatment surface as they remove.
Another currently used method for cleaning substrate surfaces without outside agents requires that the treatment surface be melted to release contaminants which are then removed by ultra high vacuum pressure. This method has the disadvantage that the surface being treated must be briefly melted which may be undesirable, as for example when a semiconductor surface is cleaned between deposition of circuit layers and it is desired that the integrity of the previously deposited layers not be disturbed. A further disadvantage with this process is that ultra high vacuum equipment is both expensive and time consuming to operate.
Annealing treatment methods suffer similar drawbacks. When a surface is cleaned by annealing methods, the treatment surface of the substrate being cleaned is heated to a temperature that is generally below the melting point of the material being treated but high enough to enable rearrangement of the material's molecular crystal structure. The surface being treated is held at this elevated temperature for an extended period during which time the surface molecular crystal structure is rearranged and contaminants are removed by ultra high vacuum. Annealing cleaning methods cannot be used where it is desired to preserve the integrity of the existing structure being cleaned.
Another currently utilized cleaning method, known as ablation, suffers from its own particular drawbacks. With ablation, a surface or contaminants on a surface are heated to the point of vaporization. Depending on the material being ablated, the material may melt before being vaporized or the material may sublimate directly on heating. With ablation cleaning techniques, if damage to the treatment surface is to be prevented, the ablation energy must be exactly aimed toward contaminants rather than the surface on which the contaminants lie, a difficult task when the contaminants are extremely small or randomly spaced. Even where the ablation energy can be successfully directed at a contaminant, it is difficult to vaporize the contaminant without also damaging the underlying treatment surface.
Surface cleaning by melting, annealing and ablation can be conducted with a laser energy source. However, using a laser energy source to remove contaminants from a surface by melting, annealing or ablation does not overcome the inherent disadvantages of these processes. For example, in U.S. Pat. No. 4,292,093, "Method Using Laser Irradiation For the Production of Atomically Clean Crystalline Silicon and Germanium Surfaces" the laser annealing method disclosed requires both vacuum conditions and energy levels sufficient to cause rearrangement and melting of the treatment surface. Other known laser surface cleaning methods involving melting or annealing require similar high energy lasing and/or vacuum conditions, as disclosed in U.S. Pat. Nos. 4,181,538 and 4,680,616. Similarly the laser ablation technique disclosed in U.S. Pat. No. 3,464,534, "Laser Eraser" suffers the same drawbacks as other high energy ablation methods.
Accordingly, it is an object of the invention to provide a method and apparatus for removing contaminants from a substrate surface that does not alter the molecular crystal structure of the surface being treated.
It is another object of the invention to provide a method and apparatus for removing contaminants from a treatment surface that does not melt or vaporize any portion of the surface being treated.
It is another object of the invention to provide a method and apparatus for removing contaminants from the surface of a substrate that introduces no additional impurities to the substrate surface.
It is a further object of the invention to provide a method and apparatus for cleaning a substrate surface that does not require a vacuum and can be conducted economically in a very short period of time.
Additional objects and advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the method and apparatus particularly pointed out in the appended claims.